The present invention relates to a plasma processing method.
Conventionally, there has been proposed a plasma etching apparatus having a lower electrode serving also as a susceptor for an object to be processed, for example, a semiconductor wafer (hereinafter called wafer) and a grounded upper electrode disposed oppositely to each other in an airtight processing chamber. This etching apparatus is composed so as to introduce process gas into the processing chamber, evacuate the processing chamber to maintain a specified decompressed atmosphere in the processing chamber, and then apply a specified high frequency power to the lower electrode on which the wafer is placed to transform the process gas into plasma, thereby etching the wafer by the plasma as specified.
There has been recently proposed a technique where processing is performed by applying to the lower electrode a power having different frequencies, for example, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-74159, a superimposed power of two frequencies by superposing the high frequency power component of 60 MHz and low frequency power component of 400 kHz. In the etching apparatus employing such a power, while generating plasma of a high density by dissociating the process gas introduced into the processing chamber by the high frequency power component, ions in the plasma are accelerated by the changes of the electric field formed in the processing chamber by the low frequency power component so as to collide against the wafer.
When etching is performed by using electric power having the low frequency power component so that the ions are accelerated by changes of the electric field, the ions get higher energy and the collision energy against the wafer is increased, and the etching rate is enhanced, but the etching area may be damaged. Also the ions have a higher potential over the grounded upper electrode and/or the inner wall of the processing chamber, and they may collide excessively against the upper electrode and/or the inner wall of the processing chamber. As a result, the inner wall of the processing chamber or various grounded members disposed in the processing chamber are sputtered, the replacement frequency of these members is increased, the throughput is lowered, and the life of the etching apparatus may be shortened.
An object of the present invention is to provide a novel and improved plasma processing method capable of performing predetermined processing on an object to be processed securely, and decreasing the replacement frequency of various members disposed in a processing chamber and extending the life of a processing apparatus, particularly by decreasing damage of the grounded inner wall of the processing chamber and the members.
To solve the problems, according to the invention, there is provided a plasma processing method for transforming process gas into plasma, in a vacuum processing chamber, by superposing and applying two different frequency power components to an electrode, and processing an object to be processed placed on the electrode by the plasma as specified, in which the lower (f2) of the two high frequency components is controlled to such a frequency that ions in the plasma cannot accelerated by changes in the electric field in the processing chamber. The lower (f2) of the two high frequency components is preferably a frequency higher than an ion plasma frequency of ions in the plasma, and a density of the plasma and a self-bias voltage are controlled by varying powers of two different high frequency power components.
In accordance with one aspect of the invention, since the second high frequency power component of the superimposed power to be applied to the electrode is set at a frequency (f2) such that ions in the plasma cannot accelerated by changes in the electric field during processing, it is possible to lower the energy of the ions produced by the changes in the electric field in the plasma, and the ion potential can be lowered for the grounded members disposed in the processing chamber, for example, the counter electrode disposed oppositely to the lower electrode, or the inner wall in the processing chamber. As a result, the energy of ions colliding against the counter electrode or the inner wall of the processing chamber is decreased, and therefore damages to these members can be decreased, and the replacement interval of the members and the service life of the processing apparatus can be extended. In the invention, meanwhile, since the ions are mainly accelerated adequately by the self-bias voltage, preferable plasma processing can be performed.
In accordance with another aspect of the invention, the most suitable density of plasma and self-bias voltage may be obtained, according to the material to be processed (an oxide film such as silicon oxide film, a conductive layer such as Al or Cu stripe, a substrate such as silicon wafer, or the like), and the type of processing (an etching, a film formation or the like).
The frequency (f2) (the lower of two different high frequency power component) is set at a high frequency relative to the ion plasma frequency of the ions which are dominant in the ion assist plasma process. Preferably the frequency (f2) is at least 2 MHz and at most 10 MHz, and more preferably at least 3 MHz and at most 10 MHz.
In accordance with the present invention, the energy of the ions produced by changes in the electric field can be reliably lowered even if in a high density plasma. Moreover, by making use of the electric power of such frequency, a high self-bias voltage can be produced on the electrode, and the ions can be accelerated in desired state. In addition, since the ion acceleration period is longer, when etching the object, for example, uniform processing can be performed securely without lowering the etching rate.
Moreover, when the higher (f1) of the high frequency power components is substantially at least 10 MHz in accordance with invention, the process gas introduced into the processing chamber can be reliably dissociated, and thus plasma of a high density can be produced, so that the object can be processed finely, promptly and uniformly. In this case, a maximum frequency (f1) may be 200 MHz and preferably 100 MHz.
In accordance with another aspect of the invention, when a magnetic field is generated in the processing chamber, plasma of a higher density can be obtained by the cyclotron motion of electrons produced by the magnetic field. Further, it has been known that in a CVD apparatus, for forming a conductive film in a fine wiring groove or contact hole, it is necessary that a desired plasma density maintains and ions are advanced onto the bottom of the wiring groove. In such cases, the present invention may be applied.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.